The predominant component of natural gas is methane. However, natural gas as produced also typically contains ethane, propane, butanes and natural gasoline (C.sub.5 +) components.
While the composition of natural gases vary significantly, the composition of a typical sweet dry natural gas stream for illustration is presented in TABLE 1:
TABLE 1 ______________________________________ Typical Sweet Dry Natural Gas Stream Component Composition, Mol % ______________________________________ Nitrogen (N.sub.2) 0.41 Methane (C.sub.1) 83.13 Carbon Dioxide (CO.sub.2) 0.50 Ethane (C.sub.2) 7.28 Propane (C.sub.3) 5.58 Iso-Butane (i-C.sub.4) 1.00 Normal-Butane (n-C.sub.4) 1.72 Pentanes Plus (C.sub.5 +) 0.38 Total 100.00 ______________________________________
The natural gas stream is typically further processed in a plant located near the production site to recover propane, butanes and natural gasoline components because (1) these heavier components are usually worth more as chemical plant feedstock than their equivalent fuel value in natural gas, and (2) to meet pipeline specifications to prevent plugging of the natural gas pipeline due to the condensation of heavier components at lower temperatures prevalent in buried pipes.
A commonly used system to treat natural gas is generally referred to as the simple refrigeration process. Of the approximately 1,000 natural gas treatment plants believed to be operating presently in the U.S., about 250 use the simple refrigeration process. In a simple refrigeration process plant, the compressed natural gas stream at a pressure of from about 350 psig to about 1,200 psig is cooled in several cooling stages to a temperature of between about 0.degree. F. and -50.degree. F. The final cooling stage is generally a refrigerated feed cooler in which the natural gas stream is cooled by a suitable refrigerant such as atmospheric vaporizing liquid propane. Other suitable refrigerants such as freon or propylene may also be utilized to achieve the required cooling of the natural gas.
As the natural gas stream is cooled, part of the ethane, propane, butanes and most of the heavier hydrocarbons are condensed. In plants located away from NGL pipeline, the condensed liquids are separated from the natural gas stream in a separator and stabilized by stripping off methane and ethane to achieve Reid Vapor Pressure of the liquid product of 250 psig maximum at 100 .degree. F. for sale via tank truck or railroad tanker. Thus, the remotely located plants generally produce a product made up of propane, butanes and C.sub.5 + mix. The operation of the stabilizer column in these remote plants is in a deethanizer mode in which ethane and lighter components are removed overhead and a C.sub.3 + product is produced.
For plants that are in close proximity to an NGL pipeline, the liquid product slate from the simple refrigeration process plant may additionally include the ethane component. Accordingly, the operation of the stabilizer column is in a demethanizer mode where methane is removed overhead because a C.sub.2 + product is produced.
Regardless of the composition of the NGL product slate, the uncondensed gases leaving the overhead of the separator are transported to market via pipeline and must meet the hydrocarbon dew point specification of the natural gas pipeline.
While the simple refrigeration-based natural gas treating plants are relatively inexpensive and simple to build and operate, they are not cost effective because they typically recover only about 15% to 30% of the ethane, only about 30% to 60% of the propane and 50% to 80% of the butanes present in the natural gas feed stream. Therefore, about 70% to 85% of the ethane, 40% to 70% of the propane and 20% to 50% of the butanes in the natural gas stream remains in the natural gas providing only the heating value credits back to the natural gas producer. Most of the time, the price of propane and butanes is significantly higher than their heating value in fuel gas. So the natural gas producers would prefer to increase recovery of propane and butanes from the natural gas feed stream by continuing to use their simple refrigeration gas treating plants. Accordingly, the low recovery of propane or propane and butane in simple refrigeration plants significantly reduces the producer's profit margin.
Sometimes the price of ethane is higher than its heating value in natural gas. At times when the price of ethane is greater than its heating value in fuel gas and if the plants are accessible to an NGL pipeline, the producers would prefer to increase recovery of ethane as well.
For the foregoing reasons, there is a need for a flexible and cost effective unit that can be conveniently retrofitted on to an existing simple refrigeration natural gas treating unit that will increase recovery of propane and butanes, and at times ethane, propane and butanes from natural gas.